When producing yarns in accordance with an open-end spinning process, the yarn is drawn out of the interior of the rotor via a withdrawal nozzle. A yarn tail turns because of the rotation of the rotor and provides the yarn with a real twist. Because of the rolling and sliding movement of the yarn over the surface of the withdrawal nozzle, a temporary twisting (i.e., a false twist) in the same direction is simultaneously generated. It is possible to improve the spinning stability during the open-end spinning process by an increase of the false twist. In accordance with the principle of the twisting compensation in the tensed yarn, both portions of the twist are propagated opposite to the yarn withdrawal direction in the yarn as far as the rotor groove and produce the transition zone thereat.
The length of the transition zone has an effect on the formation of wrap fibers, or so-called belly bands. Wrap fibers are individual fibers which have not been, or only incompletely have been, twisted into the yarn core, and which then are wound in alternating directions of rotation partially loosely, partially quite tightly, around the yarn periphery. Wrap fibers are increasingly formed by an increase in false twists, and the formation of such wrap fibers also takes place unevenly. The yarn structure suffers from this effect, and the range of application of open-end rotor yarns is limited.
It is known from German Patent Publication DE 39 34 166 A1 to affect the production of yarn in respect to spinning stability and hairiness by means of a suitable design of the withdrawal nozzle, and thereby to affect the formation of false twists. The yarn withdrawn from the rotor rolls over the surface of the orifice funnel of the yarn withdrawal conduit and receives a false twist in the process. This false twist formation is reduced respectively during and after passing notches in the yarn withdrawal nozzle, which essentially extend in the yarn withdrawal direction. In the deflecting area, the yarn is always at least partially in contact with the surface of the yarn withdrawal nozzle. Protrusions, which are arranged exactly in the center between two notches, are intended to have the effect of napping the yarn, while simultaneously providing good spinning stability. A regularly alternating contact of the yarn between the notch and the protrusion is intended to permit stable spinning on the one hand, and to cause a desired hairiness in yarn on the other hand. The yarn is plucked and thereby brought into a hairy state. Hairiness is partially caused in that outer fibers, which are not completely bound up in the yarn, are partially released from the yarn bundle and project outward. The notches or grooves cause the yarn to oscillate. The yarn oscillations are propagated as far as into the rotor groove and affect the spinning stability, the yarn structure and the yarn quality.
German Patent Publication DE 36 34 567 A1 discloses a device for drawing the yarn out of the yarn collector groove of the rotor of an open-end spinning arrangement. The surface of the yarn withdrawal nozzle, on which the yarn slidingly rolls, has sectors in the circumferential direction, in which the coefficient of friction is less than in the remaining sector or sectors. Because of the reduced friction, the yarn is given a lesser amount of false twist than outside of feeding fiber zone. Customarily, the fiber feeding zone is defined by the position of the fiber guide conduit and its orifice. The false twist is imparted to the yarn at the withdrawal nozzle. However, the wrap fibers, or so-called belly bands, are predominantly formed in the transition zone, i.e. in the rotor groove or in fiber feeding zone. The sector with the locally limited change of the coefficient of friction between the yarn and the withdrawal nozzle surface is positioned in respect to the location of fiber delivery in such a way that the reduction of false twist formation in the course of the passage of the yarn through fiber feeding zone takes place in the rotor groove.
The separation point of the yarn from the rotor groove is the location in which the yarn undergoes the greatest stress during withdrawal. The extent of yarn between the rotor groove and the deflecting zone caused by the withdrawal nozzle is subjected to greater twisting by the false twist than would result by the rotor rotation alone. The increased twisting imparted at the separation point results in an improvement of the spinning stability. Therefore the maintenance of a false twist formation is usually desired.